The invention relates to an optoelectronic laser module having a laser diode with an active region, an optical resonator with a highly silvered reflecting surface and a Bragg interference grating that provides frequency-selective feedback. In addition, a housing accommodates the laser diode and has a holder for coupling an optical conductor. Such a laser module is suitable, in particular, for use in dense wavelength division multiplex (DWDM) systems.
Laser diodes having what is termed distributed feedback are known which, because of frequency-selective feedback, unlike laser diodes with a Fabry-Perot resonator emit not in a multimode fashion but in a single-mode fashion. In particular, there are known in this regard distributed Bragg reflector (DBR) lasers in the case of which a Bragg reflector is disposed outside the normal lasing zone. This is a structure with a periodic disturbance, the Bragg interference grating, which reflects an electromagnetic wave in a frequency-selective fashion, compare the reference by Reinhold Paul, titled xe2x80x9coptoelektronische Halbleiterbauelemente), [Opto-electronic semiconductor components], Stuttgart 1992, pages 203-204.
It is also known to inscribe what is termed a fiber Bragg grating into an optical fiber. In order to generate a grating structure in this case, a reflective index is increased in specific regions of the fiber core of an optical fiber, for example by punctiform illumination of the fiber or by using a phase mask that generates an interference fringe pattern in the optical fiber. Current methods for generating a fiber Bragg grating are described in a reference by K. O. Hill et al. titled xe2x80x9cFiber Bragg Grating Technology Fundamentals and Overviewxe2x80x9d, Journal of Lightwave Technology, Vol. 15, No. 8, August 1997, pages 1263-1276.
It is accordingly an object of the invention to provide an optoelectronic laser module which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the wavelength of a laser diode can be stabilized in a simple way by a bragg interference grating, and can be set in the process to a desired value.
With the foregoing and other objects in view there is provided, in accordance with the invention, an optoelectronic laser module. The laser module contains a laser diode having an active region and a highly silvered reflecting surface, and an optical conductor having a Bragg interference grating providing frequency-selective feedback. The highly silvered reflecting surface and the Bragg interference grating form an optical resonator, the Bragg interference grating functioning as a fiber Bragg grating in the optical conductor. A housing is provided for accommodating the laser diode, the housing has a holder coupling the optical conductor.
Accordingly, the invention is distinguished in that the Bragg interference grating in the optical conductor is configured as a fiber Bragg grating, and the optical conductor is connected to the housing containing the laser diode via a holder. Because of the construction of the interference grating in the optical conductor to be coupled to the laser diode, the invention provides a laser having an external resonator, an external cavity laser (ECL). In this case, a desired wavelength of the laser diode can be set precisely by suitable selection of the grating period of the fiber Bragg grating.
In particular, by changing the pluggable optical conductor it is possible to provide a fiber Bragg grating having a different desired grating period which leads to a changed emission wavelength of the laser diode. The invention therefore permits in a simple way a setting or a selection of the emission wavelength of a laser diode, without the need to change the actual configuration of the laser diode.
Moreover, the structure according to the invention has the advantage that a very stable single-mode operation of the semiconductor laser is ensured in the case of use of quartz glass as the optical conductor material. Thus, by comparison with a semiconductor material in which the Bragg grating is constructed in the case of a DBR laser, in quartz glass the change in wavelength per Kelvin is approximately ten times smaller. Again, a larger effective wavelength is present because of the use of an external resonator. The module is therefore rendered more insensitive to temperature changes and to changes in the optical properties of the laser diode.
In a preferred refinement of the invention, the module has a coupling optical system between the optical conductor and the laser diode. The coupling optical system preferably contains a high-index coupling lens with a focal length of preferably less than one millimeter. The coupling lens is, in particular, a spherical or aspherical silicon lens, GaP lens, SiC lens or a lens made from another suitable high-index optical material. Again, use can be made of a glass lens with a particularly short focal length, in particular a glass aspheric.
The laser diode advantageously has a rear facet that is coated with a highly reflecting layer and constitutes the highly silvered reflecting surface of the resonator. A front facet of the laser diode is, by contrast, preferably coated with an antireflection layer which has a residual reflection of less than 0.1%, such that parasitic resonances of the optical resonator are suppressed. Light is emitted to the fiber Bragg grating or received thereby via the front facet, and so reflection at this facet is not desired.
In a preferred refinement of the invention, it is provided that the front facet is aligned slightly inclined to the optical axis of the laser diode, in particular at an angle of approximately 1xc2x0 to 5xc2x0. This reduces undesired feedback owing to back reflections at other structures than the fiber Bragg grating.
The optical conductor is preferably embedded in a cylindrical ferrule that can be plugged into the holder of the housing. In this case, the ferrule can be part of a fiber plug connector that can be pluggably connected to the housing.
Again, the optical conductor can be connected to a fiber plug connector in another way than via a ferrule. The ferrule or the fiber plug connector can preferably be detachably connected to the housing such that the emission wavelength of the module can be changed by simply changing the optical conductor and providing an optical conductor with a different grating period.
The housing preferably has a cylindrical holder for an optical conductor. The cylindrical holder serves to hold and couple a cylindrical ferrule that contains the optical conductor with fiber Bragg grating. Furthermore, the housing preferably has a coaxial geometry, and is preferably a TO housing. An external cavity laser with a stabilized wavelength is provided in a very cost-effective way by the use of a standard component such as a TO housing into which there is preferably plugged as a pigtail a standard fiber plug connector which only additionally has one fiber Bragg grating.
The optical conductor is preferably a single-mode glass fiber, the fiber Bragg grating being located in the glass fiber directly downstream of the end face or facet of the glass fiber. As a result, the light launched into the glass fiber immediately strikes the Bragg reflector and is consequently immediately fed back in a wavelength-selective fashion. It is possible in this case to use standard glass fibers and glass fiber plug connectors for connecting the glass fiber to the housing of the laser module, a fiber Bragg grating being inscribed into the standard glass fiber.
The end face of the glass fiber is preferably chamfered in order to avoid undesired feedback to other structures than the fiber Bragg grating.
The length of the optical resonator is preferably so short that the circulation frequency of the light is above a desired modulation frequency of the module. Otherwise, it would not be possible to transmit information on the optical information channel provided by the laser. In particular, the length of the optical resonator is preferably less than ten millimeters.
In a preferred refinement, the module according to the invention has a plurality of laser diodes, optical resonators and fiber Bragg gratings which are respectively constructed in an optical conductor, the spatial grating period of the individual fiber Bragg gratings differing for each optical conductor in such a way that light of a different wavelength is launched in each case. This provides a module with a multiplicity of optical channels such as is applied in DWDM systems, in particular. In this case, typically more than 40 channels with channel spacings of 100 GHz are implemented in a dense wavelength division multiplex.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an optoelectronic laser module it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.